If you have the tools to remove the crank arm, it might be the best solution would be to take it to a machine shop where they would know if it could be reliably rethreaded. Or rely on our old friend Sheldon Brown.
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Daniel R HicksMar 10 '12 at 14:31

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(Remember that you need a left-hand tap on the left side.)
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Daniel R HicksMar 10 '12 at 14:34

4 Answers
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Older French bicycles used a different, smaller diameter where the
pedal screws into the crank, 14 mm x 1.25 mm.

The vast majority of French bicycles imported into the U.S. used
standard 9/16" x 20 tpi crank threading, but if you find an older
bicycle that was bought in France, you may run into this problem. The
French-thread cranks can be tapped out to 9/16". This is much easier
(and more worth the trouble) with aluminum cranks. Start the taps from
the inside of the cranks,so any possible thread damage occurs where it
doesn't matter. French pedals will often be marked "G" (Gauche=left)
and "D" (Droite=right).

Yes, a pedal can be reliably enlarged and retreaded, assuming there is enough metal surrounding the original threaded hole. Most good shops have taps for 1/2" and 9/16" pedals, which are the 2 modern standards.

The older French standard is 14mm with a 1.25mm pitch.

It is possible to tap out 14mm to 9/16" without issue, if you have the tools and a steady hand.

Make sure you use the correct tap for the correct crank arm, as the left arm is reverse threaded.